It is recognized that there have been various processes for carrying out the solvent extraction of coal. Typically, coal is pulverized and mixed with a hydrocarbon solvent, with the mixture then being subjected to heat and pressure in the presence of a hydrogen gas stream. The resulting liquid mixture of solvent and dissolved coal is then separated from the undissolved coal, ash and inorganic material by settling, filtration, etc.
Also, typically, coal liquefaction has been carried out at temperatures exceeding 400.degree. C. in order to dissolve a large amount of the coal in the shortest time possible. However, processing along these lines may be looked at as being counter productive in that the basic coal chemistry and resulting chemical composition of the coal liquids is not the most desirable. For present day energy needs, we require a good distillable material that is generally free of asphaltenes and should be soluble in heptane, particularly since liquified coal, whether used as fuel oil or whether to be hydrocracked to distillate, has to be desulfurized. Without going into detail, it is well recognized that desulfurization is very difficult with asphaltenes present. They decompose to coke, gas and light distillate such that desulfurization catalysts are rapidly deactivated to have an uneconomical short life of perhaps only nine or ten months. Even at uneconomic low space velocities, the active sites of the catalyst will be covered over to reduce its useful life in a reactor.
With regard to totally liquified coals, or the so called "solvent refined coal", with a melting point of about 175.degree. C., there results only about 20 to 30% of pentane soluble material and some 50% that is not even benzene soluble. To convert this type of liquified coal to acceptable fuel takes one or more very high pressure hydrogen treating processes at extremely low space velocities such that the economics seems totally unacceptable.